Photoplethysmography, or “PPG”, is an optical technique for detecting blood volume changes in a tissue. In this technique, one or more emitters are used to direct light at a tissue and one or more detectors are used to detect the light that is transmitted through the tissue (“transmissive PPG”) or reflected by the tissue (“reflectance PPG”). The volume of blood, or perfusion, of the tissue affects the amount of light that is transmitted or reflected. Thus, the PPG signal may vary with changes in the perfusion of the tissue.
In most hospital settings, PPG is only used for measuring blood oxygen saturation, and so oximeter or multiparameter monitors are typically only configured to process the PPG signals to determine blood oxygen saturation measurements. However, the PPG signal may also vary with respiration as the PPG signal is responsive to changes in intrathoracic pressure. Thus, PPG remains a promising technique for monitoring respiration in individuals, particularly in spontaneously breathing individuals, when conventional respiration monitoring techniques such as capnometry are often not sufficiently reliable.